Canadian Journal of Earth Sciences
Musings in structure and tectonics
Journal: Canadian Journal of Earth Sciences
Manuscript ID cjes-2018-0192.R1
Manuscript Type: Article
Date Submitted by the 30-Aug-2018 Author:
Complete List of Authors: Dewey, John; University College Oxford,
Keyword: tectonics, earth sciences, geology
Is the invited manuscript for Understanding tectonic processes and their consequences: A tribute to consideration in a Special A.M. Celal DraftSengor Issue? :
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Musings in Tectonics*
John F. Dewey
University College, High Street Oxford OX1 4BH
E-Mail: [email protected]
Draft
*Submitted to Canadian Journal of Earth Sciences Special Issue: Understanding
tectonic processes and their consequences: A tribute to A.M. Celâl Şengör
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Abstract
I outline and discuss my career in the context of the history of structural geology and tectonics,
the progressive developments that led to plate tectonics, the people who have encouraged and
influenced me, the events that changed my life, my fifty six doctoral students who have taught me
so much, and my principal interests in tectonics. I discuss, in particular, nine topics of special
current interest: the evolution of Tibet, the geomorphology of the British Isles, transtension, the
Pre-Cambrian, the complexities of plate boundary evolution, Appalachian-Caledonian evolution,
ophiolites, the structure and strength of the lithosphere, and the subducting slab.
Keywords: History, memories, structure, tectonics.
Rationale Draft
This paper is unusual in its purpose and scope, and quite dissimilar to anything that I have written,
although it is something that I have thought about for years. It is a blend of my life in structure and
tectonics, the history of tectonic ideas, the people involved, and brief discussions of and opinions
on many of the topics on which I have worked and am working. The paper is designed as a brief
personal historical academic narrative. In my eighties, in “retirement”, looking back at my
academic career in geology and looking forward to the increasing myriad and complexity of things
to be found out by new generations of geologists, it seems appropriate to use this opportunity to
celebrate my student, colleague, and friend Celâl Şengör’s great contributions to geology by writing, in this volume in his honour, a brief account of my views about some the things that my students have taught me and that have fascinated me over my sixty five years in geology. I hope that the reader will forgive me if my memory of details is less than perfect but I have tried to restrict myself to those events and ideas that I recall with reasonable clarity.
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Early years
During my years (1948-1955) at Bancroft’s School in Woodford Green, Essex, my
chemistry master John Hayward, an amateur geologist, sparked my interest in geology by
involving me and several other teenagers in his research on the Holocene stratigraphy and palaeo-
environments of the Lea Valley and by organizing three week summer bicycle tours of many parts
of the UK and Ireland, the most memorable of which were Devon and Cornwall, the Lake District,
the Northwest Highlands, and Ulster. During these tours, we examined an enormous variety of
rocks and their relationships, scenery, history, and geomorphology. I then read geology at Queen
Mary College University of London (1955-1958), which had a small geology department known
for its excellent teaching of basic geology.Draft I was not disappointed. The four members of staff led
by J. F. Kirkaldy (Kirk) were dedicated to the teaching and welfare of the five students in each
year. Knowledge gained during the school bicycle tours was a fine foundation for the intense
course in classic basic geology, firmly rooted in the field. We did two two-month pieces of
independent mapping, one soft and one hard-rock; mine were the Osmington-Ringstead-Poxwell
area of Dorset, where I became proficient at mapping from spring lines, changes in slope,
vegetation, and auguring to three metres, and Cader Idris in North Wales, a treasure trove of
extrusive and hypabyssal Ordovician volcanic rocks. We also had three-week Easter field courses
in the Welsh Borderland, the Scottish Highlands, and Northern Ireland, and many weekend trips
into the Weald, to Shropshire and North Wales. On a field trip to northeast Scotland, we were
puzzled by the Helmsdale Fault-bounded mainly clastic Jurassic sequence with boulder beds and
fallen sea stack; in the 1950’s, we had no knowledge of Jurassic rifting in the North Sea from
drilling and seismic sections. The highlight and most valuable feature of the whole course was that
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we were given, every week, a one-inch geological map of some part of Britain, from which we had to analyse the structure with sections and block diagrams, the stratigraphic history, unconformities, and all other relevant features, and write a report on the structure and history of the area solely from information on the map. Maps of Ireland, other European countries, and North
America. at various scales, were also used. In this way, the whole course was drawn together while learning a huge amount of basic geology. I was very fortunate and am deeply grateful to Queen
Mary College for providing me with this meticulous grounding in geology and report-writing. The only deficiency, typical of almost all geology courses world-wide then, was that there was little or no attention paid to larger-scale features of the earth at tectonic scales. Continental drift was mentioned, as were Van Bemmelen’s ideas of tectogenes and Stille’s on geosynclines but these were presented as quite unrelated to theDraft geology that we were learning. In London intercollegiate lectures, Stanley Hollingworth outlined the idea of continental drift and remarked that it was obviously correct but seemingly irrelevant to continental geology. Lester King, the great South
African proponent of continental drift gave some lectures in London that convinced most of us of its reality. Thus, the hypothesis was accepted, at least in London, by most geologists but tucked away as a curio. The only foray into the broader picture were Kirk’s stratigraphy lectures in which he gave us correlation charts for each System in Britain and encouraged us to colour them with lithologies , in as much detail as possible, and try to explain the facies distribution and likely palaeogeography, such as the carbonate platforms and shale basins of the early Carboniferous of the Pennines. On one occasion, another student, Philip Drummond, and I plotted up the age pattern of the Ordovician-Siluian turbidites of the Southern Uplands and showed that the turbidite package becomes younger southward; the concept of subduction-accretion above a northward-dipping
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subduction zone was then completely unknown and un-thought of. However, the principle and
value of tectono-stratigraphic charts was seeded in my mind.
Following Queen Mary College, I was accepted by Imperial College to do a Ph.D (1958-
1960). In those days, a Ph.D in geology generally involved experimental petrology, palaeontology,
or being given an area to map and discover what geological secrets it held. I was offered a menu
of carbonate biostratigraphy in the Jura (Derek Ager), Moine stratigraphy and structure in the
Northwest Highlands (John Sutton), and Lower Palaeozoic rocks in the Sheeffry Mountains of
western Ireland (Gwyn Thomas). I chose western Ireland for its rather exotic ambience, its very
wide variety of lithologies and structures, and that it had last been mapped regionally in the 1870s.
Also, William Stanton had just completed an Imperial College Ph.D on the rocks of southwest
Murrisk immediately west of the SheeffryDraft Mountains, which provided a useful standard and
framework upon which to hang my work. Also, I felt that, in the Highlands, I would be a mere cog
in the Moine machine and, in the Jura, I might be committing myself to a lifetime in carbonates.
Western Ireland, apart from the weather and flies, was Geo-Utopia. I spent about seven
months mapping about four hundred square kilometres of Ordovician and Silurian rocks from
Croagh Patrick, through the Sheeffry Mountains to the Partry Mountains. I mapped turbidites,
lahars, ash-fall tuffs, ignimbrites, cross-bedded red-brown fluviatile sandstones, shallow marine
sandstones and conglomerates, all variably and polyphase-deformed in the greenschist facies.
Fossils are scarce in these rocks, graptolites in the Ordovician, brachiopods and corals in the
Silurian. My time in London was spent with thin sections, reading extensively, and buttonholing
and learning as much as I could from a distinguished academic staff, including Ian Carmichael,
Graham Evans, John Ramsay, Doug Shearman, George Walker, and Janet Watson, who gave their
time freely.
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Since my Ph.D work, western Ireland has been my principal geological home. I have learned and honed field observational skills there, and it has been an inspiration in much of my work in tectonics. It remains a treasure chest of wonderful geology that has inspired many geologists, and is a key to Appalachian-Caledonian evolution (Dewey, 2005; Dewey and Mange,
1999; Dewey and Ryan, 2015). Paul Ryan and I are still working on the South Mayo Trough even after over a hundred years of combined years of work. Currently, we are writing a synthesis of its structural evolution as a late Silurian transpression zone. Of course, in 1960, we had no way of knowing the origin of the South Mayo Trough as a fore-arc basin obducted northwards across the edge of Laurentia during arc- continental margin collision but much of the data that I collected in the 1950s has been of great value in recent syntheses. In my Ph.D viva, Dan Gill asked me what I thought about the larger-scale origin ofDraft the South Mayo Trough and the Dalradian Barrovian metamorphic rocks of western Ireland. I replied that they were, somehow, part of an Ordovician eugeosynclinal volcanic tract along the edge of the Laurentian craton. John Sutton interrupted with the acid comment that this was speculation, which should be practised only by more experienced and senior people. I subsided and bided my time
Manchester (1960-1964)
On gaining my Ph.D, I accepted, from Alex Deer, the position of Assistant Lecturer in the
University of Manchester rather than a similar position in the University of Liverpool offered by
Robert Shackleton because I was told that the Liverpool Department was a poorly-organized shambles. I soon realized that I had made a serious mistake in following poor advice. With my field interests in the Caledonides, I found the Manchester Department extremely dull, immersed as it was in mineralogy and the experimental petrology of quartz-feldspar systems. The only person
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with whom I could chat enjoyably about structural and regional geology was Robin Nicholson, a
very fine field-based structural geologist working in Norway. Therefore, still influenced by John
Sutton’s admonition, I immersed myself in writing up my thesis work and researched the structural
geology of kink bands in crystals and foliated and banded rocks. I also discovered the joys of
lecturing and leading field trips. Few of the academic staff, apart from Robin Nicholson were
interested or competent in organizing and leading field courses so I acquired a substantial amount
of field instruction. Each year, I took the five or six third year students to Assynt for three weeks,
where we walked across some of the most spectacular geology on Earth, using, as our guide, the
Peach and Horne 1907 Northwest Highland Memoir and map, and jointly mapped a small area in
great detail. On two of these trips, we walked the thirty kilometre Kinlochewe to Lough Broom
traverse along the Moine Thrust Zone, madeDraft by the young Archibald Geikie in 1860, and marvelled
at how a twenty five year old professional geologist could make so many serious errors of
observation and interpretation. Robin Nicholson was senior to me and did all the structural
teaching, while, as a dogsbody, I was given courses that nobody else could or would give such as
vertebrate palaeontology, geomorphology, the amphiboles, and geophysics but the first year
introductory course was an especial pleasure into which I threw myself vigorously. My boredom
in Manchester was relieved by frequent weekend trips to Liverpool and field trips to North Wales
with Robert Shackleton, Nick Rast, and Dennis Wood. Life in the Manchester Department took a
positive turn when David Vincent, an amiable and intellectual man, was appointed Professor and
Head of Department and Alex Deer left for Cambridge.
My life in Manchester was greatly enlivened when structural geology underwent a major
transformation by the publication of Derek Flinn’s (1962) brilliant and now classic paper on the
deformation ellipsoid. The Geological Society of London meeting at which Derek presented his
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paper was crowded and electric. The powerful Flinn diagram was introduced as a means of understanding the rotation of lines and planes, fields of shortening, reduced shortening, and extension, and lines and planes of no infinitesimal and no finite deformation. Many things began to make sense such as “chocolate tablet” boudinage and polyphase deformation in a strain continuum. However, Flinn’s analysis was fatally incomplete because of his aversion to shear and vorticity in rock systems. Hence, deformation, for Flinn, was entirely coaxial and deformation paths were straight-linear, and K-constant whereas, apart from pure oblate and prolate and coaxial plane strain, in reality, all paths in nature must be non-coaxial with changing K values and even axis switching in transtension or transpression. I remember tense discussions with Derek about narrow high-strain zones; for compatibility reasons, non-coaxial strain must occur in any deformation zone between non or less-deformingDraft blocks.
In 1961, my first Ph.D student, Bill Morton, started mapping Errisbeg Mountain, one of several calc-alkaline gabbroic intrusions in Connemara, later interpreted (Wellings, 1998) as syn- kinematic asthenospheric partial melts of the arc that was obducted across the Laurentian margin and responsible for the “hot iron” Barrovian metamorphism of the north-verging Dalradian nappes
(Dewey and Ryan, 2015). Deer refused any financial support of Morton’s field work; expenses were covered from our own personal funds. Sadly, Bill Morton was shot in Ethiopia only a few years later. Andrew Tinnion completed an M.Sc. on the sedimentology of the Arenig Garth Grit in North Wales and linked it to the early Ordovician great orthoquartzite blanket of Brittany.
I met Stuart McKerrow in 1959 while mapping in western Ireland. Stuart and his student
Colin Campbell had mapped a huge tract of Silurian rocks in north Connemara and we began a lifetime friendship doing geology in Galway and Mayo together, We made a geomorphological analysis of Galway and Mayo, coming to the conclusion that, apart from glacial modification of
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the landscape, Cenozoic faulting of a sub-Carboniferous surface had generated western Irish
topography, rejecting the resequent hypothesis. We also mapped a strip of Silurian and Ordovician
rocks from the Maam Valley through the Lough Nafooey Valley along what is now believed to be
an early Ordovician oceanic arc. We planned to continue the mapping through Tourmakeady along
the eastern side of the Partry Mountains but were warned off by Alwyn Williams who claimed to
have completed mapping that tract; the map never appeared, illustrating the maxim that terrains
belong to no-one and to ignore “warnings off”.
Another agreeable interval was in 1963, when the Geological Survey of Ireland invited me
to map the coastal volcanic rocks of Co. Waterford from Annestown to Tramore, and record any
economic mineral shows. Nick Rast who, with his students, had done inceptive work on the
Snowdon volcanic centre spent some Draft time with me and I learned a great deal of physical
volcanology, especially on lahars and peperites.
My introduction to the North American continent came in the summer of 1964. Art Boucot
and Stuart McKerrow asked me to spend a month with them mapping the Silurian rocks of Arisaig,
Nova Scotia (Boucot et al, 1974). Marshall Kay heard of the invitation and persuaded me to spend
a month with him in Newfoundland based in New World Island in Notre Dame Bay. Nova Scotian
and Newfoundland geology excited me greatly and I realized that I was about to enter a major new
phase of research that combined mapping with thinking about the Appalachian-Caledonian
Orogenic Belt at a much larger scale. In Newfoundland, I met Bob Stevens in the fledgling
Memorial University. Bob, Werner Brueckner, and Hugh Lilly were mapping and beginning to
understand the Taconic allochthon of Humber Arm and its cap of the Bay of Islands Ophiolite
Complex.
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Cambridge (1964-1970)
My appointment to a Lectureship in Cambridge in 1964 broadened my interests and provided new opportunities. The appointment mechanism, might be considered unusual today. I was interviewed very casually by Oliver Bulman for about thirty minutes, taken to lunch in Sidney
Sussex College, and offered the job. Although I was the only structural geologist in the
Department, there were many very interesting people in the three earth science departments,
Geology, Mineralogy and Petrology, and Geophysics, and in the other science and engineering departments. I began to think of geology in a multi- and inter-disciplinary context and attended many lectures in the other science departments. Life in Trinity College and later in Darwin College was also intellectually agreeable. I learned a great deal from Alan Smith, Martin Rudwick, Peter
Friend, Barrie Rickard, Graham Chinner,Draft and Teddy Bullard. Teddy Bullard became a valued friend and adviser, telling me not to do the research that senior people wanted done, to follow my own instincts and interests, and work across a wide range keeping broad interests. However, a key to my metamorphosis in Cambridge was the range and variety of sabbatical visitors to the three earth science departments, such as Harry Hess, Charles Drake, Jack Nafe, and Tuzo Wilson,
I arrived in Cambridge, as the “house-pet” structural geologist, in the middle of a revolution in which tectonics was evolving rapidly as a logical and actualistic subject. Until 1965, I was immersed in structural geology and the geology of western Ireland. David Piper and Ben Dhonau in Ireland, and Brian Lynas in North Wales, completed fine Ph.Ds with me. I spent wonderful summers mapping in the South Mayo Trough and spending time looking at rocks in western
Ireland with Adrian Philips, Ben Kennedy, Charles Morley, and Robert Shackleton.
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History of tectonics
It is appropriate, at this point, to outline and discuss the gestation of tectonics as a
background to the new world that I encountered in Cambridge. I will outline, through the key
people, the rather slow but logical evolution of ideas that led from Alfred Wegener and continental
drift, to Tuzo Wilson and plate tectonics and its role, during the late1960s and early 1970s, in
generating the critical framework to explain a great deal of geology. It is inappropriate, here, to
relate the whole history of tectonics in great detail as there are many very good accounts. Rather,
I will outline, from my perspective, the key people, many of whom I knew or know, and their
observations and ideas. Some of these people are lesser known and were or are geologists working
in the field who deserve greater credit than they have received.
Alfred Wegener (1922) proposedDraft the then seemingly outrageous notion of Continental
Drift; that the continents around the Atlantic Ocean once fitted together and that they drifted into
their present relative positions. The congruent shapes and fit of the continental margins of the
Atlantic had been noticed by Snider-Pellegrini (1858) and was further developed by Van
Waterschoot Van Der Gracht (1926), Choubert (1935), Du Toit (1937), and King (1951).
Choubert’s reconstruction of the continents around the Atlantic is almost identical to that of
Bullard et al (1965). Apart from the geometric fit of rifted margins, continental drift and the growth
ocean basins is irrefutably demonstrated by the detailed matching of geological patterns on
congruent margins (Choubert, 1935). Wegener (1929) recognized that a rotation pole in Alaska
and rotation angle can be used to define the relative motion of North America and Europe, thus
implying torsional rigidity of the continents.
Continental drift was not only a major bone of contention for fifty years but even many of
those who accepted the idea could not see a way in which the theory could explain the geology of
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Earth. Émile Argand (1911) was an exception; following Heim (1878), Bertrand (1884), Callaway
(1883, 1884), Lapworth (1883, 1885), Lugeon (1901), and Peach and Horne (1907), he recognized that mountain belts commonly show very large amounts of horizontal shortening and thrusting.
Ampferer and Hammer ((1911) showed, in the Alps, how large amounts of continental shortening are effected by the underthrusting and swallowing of continental crust in “Verschluckungszonen”, a process later termed “subduction by Amstutz (1951). Argand (1924) concluded that the
Himalayas were formed by the collision of peninsular India with Asia following the closure of an ocean (Tethys) and involved major underthrusting of the Indian subcontinent beneath Tibet and the raising of the Himalayas and Tibet. Thus, by 1929, the idea was current that continents could split, drift to open and close oceans, and collide to form mountains, and that relative motion among torsionally-rigid continents could be describedDraft by rotations.
Wadati (1932), Berlage (1937), and Benioff (1949) recognized the inclined zones of earthquakes, now called Benioff Zones, sloping beneath volcanic arcs from oceanic trenches and indicating that slabs of a stiff/brittle material penetrates the mantle from an oceanic source. Plafker
(1964), nailed the concept of oceanic subduction in his brilliant mapping of the geomorphological effects of the 1964 “Good Friday” 9.2 Alaskan earthquake with an epicentre 25 km beneath the northern end of Prince William Sound. At the epicentre, the two nodal planes intersected along a roughly trench-parallel line from which the steeply south-dipping nodal plane projected to the surface for over 600 km and separated an area of uplift to over 2.5 m to the south from an area of subsidence to 2.5 m to the north. Press and Jackson (1965) regarded the steep nodal plane as a high-angle thrust, extending to a depth of over two hundred kilometres and generating the earthquake. Plafker argued, cogently, that this must be incorrect because it would involve a fault with a 600 km strike length slipping at 25 km depth without breaking the surface, a kinematic
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impossibility. Plafker showed that the other, gently-dipping nodal plane must have been the
earthquake-generating thrust fault dipping northward beneath the Alaskan arc and projecting, via
a suite of aftershocks to the base of the inner wall of the Alaskan Trench. Thus was established the
underflow or subduction of the Pacific Ocean floor beneath the volcanic Alaskan Arc, the first
clear proof of subduction and a key element in the development of plate tectonics.
Between the wars, Holmes (1931) and Griggs (1939) developed a broad, rather loose,
cross-sectional picture of continental splitting, oceanic growth, subduction and continental
collision in relation to mantle convection without specifying the rheologies involved because so
little was known about the geology of the ocean basins. Stille (1924), although a fixist who made
many errors, recognized zones of thick sediments and volcanics (geosynclines) in deformation
zones that contrast with the epicontinentalDraft platforms. Kay (1951) developed the geosynclinal
theory much further by recognizing a wide variety of sediment and volcanic associations with
particular relationships with orogenic belts. In particular, the miogeosyncline-eugeosyncline non-
volcanic-volcanic couple, which he had learned from Stille’s (1940) great, but little-read book
Einführung in den Bau Amerikas, mimics the continental shelf/margin/rise and outboard oceanic
assemblages and arcs. Kay was always insistent that his geosynclines are descriptions of
stratal/facies assemblages and should not be seen as actualistic linear furrows. Dewey and Bird
(1971a) set Kay’s geosynclines into a plate tectonic framework. Vening Meinesz et al (1954),
famous for the invention of the bipendulum swing gravimeter, enabling gravity measurements at
sea, discovered the fundamental association of trenches and volcanic arcs with, respectively,
negative and positive gravity anomalies, to be explained later by the subduction of oceanic
lithosphere. Thus, until about the second world war, a general pattern of continental drift with large
displacements was current but lack of oceanic data prevented the development of a coherent
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kinematic theory. Also, the theory was not widely accepted, especially in the US, and its relevance to geology was not obvious. The concept of the lithosphere, a stiff outer Earth layer and the stuff of plates, had been propounded already by Love (1911) and developed by Barrell (1914). Anderson
(1962) showed that a very slow S-wave layer, the low velocity zone, lies below a lithosphere up to 200 km thick. Also, torsional coherence of the lithosphere, if not rigidity, was implicit in the fit of rifted continental margins. The implications of these two features were not discussed and incorporated until 1965.
Great advances came with the discovery of very large displacements on very long strike- slip faults that must penetrate the brittle lithosphere. Wellman (1955) discovered the Alpine Fault and its Miocene to Present 500 km dextral offset in South Island, New Zealand and, presciently, noted that the system connects the HikurangiDraft and Puysegar Trenches, a harbinger of Tuzo Wilson’s transforms. Harry Wellman was a brilliant iconoclast and outstanding structural thinker. On sabbatical leave in Victoria University in Wellington in 1992, I was regaled, once a week over coffee, with his mapping exploits in the South Island. In his house in Kelburn, I was shown many superbly-built models in wood, metal, and card illustrating a wide variety of structural and tectonic phenomena, including a clever trench to trench elongating transform that he built after he discovered and mapped the Alpine Fault. He had, also, models of transcurrent faults with restraining and releasing bends with one wall made of rubber to illustrate bending, rotation, strain, and unbending as that wall moved through the bends. Kennedy (1946) argued for 100 km of sinistral displacement on the Devonian Great Glen Fault which is likely the continuation of the
Cabot Fault in the Canadian Maritimes (Wilson, 1962); the offset along this system is probably up to about 500 km (Dewey et al. 2016). Hill and Dibblee (1953) outlined the essentials of the San
Andreas Fault in California, noting its 500 km dextral offset and its termination at Cape Mendocino
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and in the Sea of Cortez. Of especial importance is Quennell’s (1958) analysis of the Dead Sea
Fault as a 112 km sinistral displacement transcurrent fault forming a small circle of rotation around
a pole near Gibraltar, and ending at the Maras triple junction and the Red Sea oceanic opening,
implying torsional rigidity and tectonic zone (plate boundary) linkage. This analysis was the first
glimmerings of plate tectonics sensu stricto but remained a “sleeper” for ten years.
The next link in the chain came with Irving and Runcorn’s (1957) use of the new Blackett
magnetometer to show that India had moved north by 6,000 km and rotated by more than 30°.
They also showed a major discrepancy between the pole from modern Earth and that for the Neo-
Proterozoic Torridonian strata of northwest Scotland. Creer et al (1958) showed that the Mesozoic
APW paths for North America and Europe almost precisely coincide when the North Atlantic is
closed indicating not only the opening Draftof the Atlantic during the Cenozoic but also the torsional
rigidity of North America and Europe during that opening.
With the data that came from the post-WWII international oceanographic explorations of
the great American oceanographic institutions led by the luminaries Maurice Ewing, Roger
Revelle, and Bill Menard, the general topography of the floors of the worlds oceans, was
discovered especially the broad symmetrical oceanic ridges. Heezen (1960), with Marie Tharp,
pioneered the depiction of the bathymetry of the world’s oceans in a series of maps, which showed
the ridges, their fracture zone offsets, trenches, guyots, sea-mounts, sea-mount chains, oceanic
plateaux, and rifted margins. The smooth abyssal plains were demonstrated, by sparker seismic
profiles, to be huge areas of flat -lying sediment burying a rough-relief basement and progressively
thinning on the ridge flanks to zero at the ridge crest. Shallow drilling showed the sediments to be
lutites, cherts and carbonates with occasional sandy turbidites near the continental margins,
extending far out along fracture zone troughs. The Lamont core lab contains a treasury of these
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early cores. Dredging, especially along fracture , indicated that the basement is an assemblage of mafic and ultramafic rocks. This phase of marine exploration included the collection of a huge amount of gravity and magnetic data. Therefore, by about 1960, much of the data base with some key ideas were in place, like those of Wellman (1955) and Quennell (1958 that were to lead, like an unleashed flood, to the coherent theory of plate tectonics during the 1960s.
In the early 1960s, there was still a substantial divergence of opinion on the veracity of continental drift, mainly between Harry Hess and Bob Dietz on the one hand who embraced the theory and developed the idea of sea-floor spreading by axial ridge accretion as a mechanism for the growth of the ocean floor, and a group of fixists, including Maurice Ewing and Harold Jeffreys, who espoused the permanence of ocean basins. Maurice Ewing was deeply influenced by his colleague Walter Bucher who did not Draft“believe” in thrusts, nappes, and crustal shortening, and whose judgment on most geological matters was deeply suspect from arguing a cryptobleme
(endogenic) origin for astroblemes (bolide impacts) to an interpretation of the Taconic allochthon as a clastic basin in the Stockbridge carbonate shelf. Ewing opposed mobilism until his death in
1974. Harold Jeffreys argued against continental drift on the basis that it is physically impossible for weak continents (quartz) to plough through strong oceans (olivine), a correct proposition but not the way in which continental drift happens. Bruce Heezen and Sam Carey accepted sea-floor spreading but not subduction and were, therefore, forced into earth expansion, which is demonstrably incorrect from palaeomagnetic triangulation. Almost until his death in 2002, Carey gave extremely entertaining and polemical lectures, around the world, of up to three hours on the expanding Earth. Carey made some important observations and had harbinger ideas (Carey, 1954,
1955, 1958) on, for example, the anti-clockwise rotation of Iberia, earlier suggested by Wegener
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(1922) and Argand (1924), the resultant opening of the Bay of Biscay “sphenochasm”, and hotspot
tracks, or “nemataths.”
The mechanism, kinematics, and rates of sea floor spreading were formulated following
the discovery of the “zebra-stripe” pattern of positive and negative magnetic anomalies off the
Pacific coast of Washington and British Columbia (Raff and Mason 1961) and on the Reykjanes
(mid-ocean) Ridge south of Iceland, the latter obviously ridge-parallel and symmetrically paired
across the Ridge. Later, a similar parallelism and symmetry was seen across the Gorda Ridge.
Morley and Larochelle, and Vine and Matthews (1963) outlined the now-familiar magnetic tape
recorder theory of the development of TRM in successive strips of mafic oceanic crust accreted
symmetrically at a ridge axis during alternating periods of normal and reversed polarity. Pitman
and Heirtzler (1966), using the Eltanin Draft19 profile, extended the concept across the fast-spreading
East Pacific Rise. By 1985, the age of the whole oceanic lithosphere of Earth and spreading rates
from 160mma-1 on the East Pacific Rise to super-slow on the southwest Indian Ocean Ridge were
established. George Walker arrived at the same conclusion from mapping in Iceland. He realized
that, as mafic dykes feed basaltic flows, the number of dykes increases downward and that, at a
few kilometres, the mafic crust must consist of nothing but mafic dikes, a theoretical recognition,
in the mid-fifties, of sheeted dyke complex and sea floor spreading. He also calculated, from the
age spread of the basaltic section, a spreading rate of 20 mma-1, identical to that derived from
magnetic anomalies on the Reykjanes Ridge. He taught this, modestly, in his undergraduate
courses and small seminars in Imperial College.
The scene was now set for two breakthrough papers that implicitly and explicitly
respectively, discovered plate tectonics. First, the critical importance of Bullard, Everett ,and
Smith (1965) has been described by Dewey (2015) and may be briefly summarized as follows.
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1. Restoration of the continents around the Atlantic was achieved by minimizing the misfit
between edge of the continental shelf at the 50 fathom line, about 900 m).
2. Torsional rigidity of the continents was assumed because distortion of continents and their
margins would prohibit fitting. The superb fit demonstrates torsional rigidity.
3. Fitting of continental margins was done, using Euler’s Theorem, by finite difference
angular rotations around axes passing through the centre of Earth intersecting Earth’s
surface at poles of rotation.
4. Rifted continental margins may be very sharp with a rapid transition from continent to
ocean, the ideal margin that fits cleanly. Overlaps were explained mainly as the growth of
deltas since continental separation, voluminous basaltic lava fields, and continental
stretching rather than a sharp break.Draft
5. The recognition that smaller complicated areas like the Caribbean and Gulf of Mexico need
special geological analyses to unravel the behaviour of small blocks. Some small
continental blocks, such as Iberia, have rotated by up to 40° during periods as short as
10Myr around nearby rotation poles to open small, wedge-shaped oceans such as the Bay
of Biscay.
6. When the continents around the Atlantic are restored to a ‘Pangea configuration’ there
appears the huge, westward-narrowing and terminating gap of the Tethyan ocean complex,
which closed as a result of the Central and North Atlantic opening at different times around
different poles of rotation.
Secondly, the theory of plate tectonics was formulated by Wilson (1965). He coined the term “plate” as almost earthquake-free areas bounded by continuous earthquake zones that form a
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mosaic of three kinds of boundary, ridges where lithosphere is generated, trenches where it is
subducted and strike-slip transforms; three boundaries join at a triple junction. Transforms may
end at triple junctions or change orientation to become ridges or trenches. This solved a major
problem in the oceans; transform offsets of the ridge axis are not fault displacements of the ridge
but simply offsets so that, for example, a transform offsetting a left-stepping ridge axis is dextral,
the separation rate at the ridge axis equalling the displacement rate along the transform. This
explained why earthquakes terminate at the ridge terminations and do not continue along the
aseismic fracture zone continuation. This was tested by Sykes (1967), using seismic first motions,
who showed the transform model to be correct. Oldenburg and Brune (1972) showed
experimentally, in a hot wax “lithosphere”, how ridge to ridge transforms may originate by joining
offset propagating cracks. Tuzo WilsonDraft was developing his plate tectonic theory during his time
during 1964-1065 as a sabbatical visitor in Cambridge where I was a young lecturer. I had the
privilege of his regularly coming into my room for coffee and expounding his many ideas from
plate tectonics to seamount chains and the Appalachian-Caledonian belt (Wilson, 1966). He
produced his now-famous paper model of an evolving ridge to ridge transform system; I was
transfixed and spent many fascinating hours with him in my room in the Sedgwick Museum cutting
out card shapes of simplified parts of Earth and letting them evolve by sliding the pieces of card
around. We evolved seven kinds of transform, two stable and two unstable, and seven types of
triple junction. I still have the pieces of Bristol Board. Two of these, in continental crust in
particular, I remember as revealing but then undiscovered. First, we considered where three large,
same-sense displacement transforms join in a continent; of course either a hole opens up or the
lithosphere is radially stretched. The second is a transform-bounded wedge driven from a
convergent zone, which produces the field paradox of dextral and sinistral walls of the wedge
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being transposed together in the shortening zone giving apparently conflicting slip senses (Fig.1).
Unknown to us then was the work of Ketin (1948) who was the first to suggest that an Anatolian block was moving westward with respect to its surroundings along narrow boundaries (the North and East Anatolian Faults while maintaining a high degree of internal rigidity. Subsequently these hypothetical relationships were discovered as the Adana and Karliova junctions when Şengör was writing his paper, as a final year undergraduate at Albany, on the North Anatolian Fault (Sengor
1979a) showing how an arrowhead-shaped hole must open where the North and East Anatolian
Faults meet. The development of these triple junctions was developed by Dewey et al (1980, Figs
14, 15, and 18); Sengor and I are now working further on the geology of these two triple junctions.
McKenzie and Morgan (1969) published a thorough analysis of the theory of triple junctions. Of course all this was by translation on a flatDraft surface but Tuzo knew both that Earth is a sphere and realized that transforms formed small circles around rotation poles. Ten years later, he remarked to me and Kevin Burke that, had he known Euler’s Theorem, he would have had the complete theory in his 1965 Nature paper. At about this time, R. A Lyttleton and Harold Jeffreys warned me, albeit in friendly fashion, that I would do my career no good by becoming further involved with the rapidly-developing mobilism. Teddy Bullard told me to politely ignore them.
Cambridge and North America
Tuzo Wilson changed my academic life by encouraging me to add a tectonic scale to my structural geology interests. I did this, enthusiastically, following a 1965 field conference in Nova
Scotia and Newfoundland, with Ward Neale, John Rodgers, Hank Williams, and Phil King, by beginning a synthesis of the Northern Appalachians and the Irish and British Caledonides using, as my base, a huge roll of drafting linen. In 1966, I attended the Goddard Conference in New York
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on continental drift where I presented a paper with Marshall Kay on the fit of the Appalachian and
Caledonian Belts (Dewey and Kay, 1968) followed by a field trip with Marshall, and Dan
McKenzie, on the northern end of the Taconic Allochthon in Vermont. In 1966 and 1967, I spent
the summer months mapping in Newfoundland, and on sabbetical in Lamont, where I analysed
and synthesized all the Appalachian State and Provincial geological maps in the Columbia library,
gradually generalizing and transferring the patterns to my tracing linen roll. I also toured the
universities and colleges of New England where many people, such as Ed Belt, Peter Robinson,
Brad Hall, and Don Wise gave generously of their time and information, and showed me the rocks.
I coloured the map, as best I could, including facies and structural patterns and ages as
follows; carbonate platforms and allochthons upon them, rift complexes, rift volcanics, continental
margin facies transitions, metamorphicDraft complexes and their facies, volcanic arcs and their
petrology, sutures, ophiolites, major unconformities, nature and age of intrusions in the upper half
of the Streckeisen diagram, and patterns and age of deformation. The first draft of the map was
completed, by Christmas 1967. The general patterns immediately became clear and the map
became the basis for a series of papers on the evolution of the Appalachian-Caledonian belt, and
mountain belts in general (Dewey, 1969, Dewey and Bird, 1970).
By the end of 1968, the general theory of plate tectonics was tidied up and almost complete
(McKenzie and Parker, 1967; Morgan, 1968; Isacks, Oliver, and Sykes, 1968; Le Pichon, 1968).
Subsequently, assemblages of instantaneous rotation poles were derived for the whole Earth
(Chase, 1972; Minster et al, 1974; De Mets, 1993 ).
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Plate tectonics and geology
An outline of the geological “clothes” of plate tectonic was developed in the late 1960s and early 1970s for explanations of the geology of rifted continental margins, island arcs, collision zones, and the diversity of igneous and metamorphic rocks (e.g. Coney, 1970; Church and Stevens,
1971; Hamilton, 1969, 1970; Dewey, 1969; Dewey and Bird, 1970, 1971b). The Asilomar Penrose
Conference in California on Plate Tectonics and Geology was convened by Bill Dickinson in
August 1969. The participants ranged from senior distinguished geologists such as Jim Gilluly,
Warren Hamilton, and John Rodgers to recent graduates such as Steve Delong, with the bulk in their thirties, such as me, Clark Burchfiel, Greg Davis, Eldridge Moores, and Dietrich Roeder. The meeting was the most exciting and productive of my career. I made a host of friends and realised that I needed to be in North America forDraft the next phase of my life, which happened in January
1971 when, in spite of offers from Princeton and UCLA, I moved to the fledgling geology department in the State University of New York at Albany because of gentle, friendly pressure from Jack Bird, advice from Art Boucot and John Rodgers, and Albany’s position adjacent to the marvellous geology of New England.
Suny Albany and Lamont
In January 1971, I arrived in Albany to take up my position as Professor of Geology.
Almost immediately, Jack Bird and I set about filling four positions released by the Dean to build the core of an enhanced Department with a graduate program, We were fortunate in hiring Jeff
Fox, Steve Delong, Akiho Miyashiro, and Fumiko Shido. Win Means, Peter Benedict, and George
Putman were already members of the Department. Jack Bird left for Cornell in 1972 and we hired
Kevin Burke, and Bill Kidd. We began to attract a series of outstanding graduate students from the
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best four year colleges in the Northeast and an outstanding Department was born. We were an
academic family that lasted ten years in a utopian academic Camelot during which faculty and
students researched together with no barriers in a Department that buzzed from early morning to
late evening. Except for Win Means’ experimental structural research and Akiho Miyashiro’s
petrological-geochemical research, our research and Ph.D programme was largely field-based in
New York, New England, Newfoundland, the Caribbean, Turkey, and Switzerland, combined with
the compilation and analysis of global continental and oceanic data as the basis for many syntheses
of tectonic environments and mechanisms, crustal evolution through time, and regional geological
syntheses. Our approach was data-heavy and involved very little numerical modelling but a great
deal of kinematic modelling. Our graduate students worked tirelessly with us and produced
original and outstanding work. Our topicsDraft were eclectic and ranged across most plate boundary
zone environments: ophiolites and oceanic lithosphere generation at ridges and fracture zones, arc-
continent and continent-continent collision, continental transforms, subduction-accretion, and
continental rifting.
Ophiolites
Associations of serpentinite, spilitic pillow lava, and chert, were recognized as an early
deep-sea component of orogenic belts by Steinmann (1926), especially in the Alps and Apennines
and named the Steinmann Trinity. The Trinity was absorbed gradually into the umbrella of
ophiolites and ophiolite complexes. Gass and Masson-Smith (1963) came very close to a sea-floor
spreading followed by obduction origin for the Troodos Massif in Cyprus, arguing that the mafic
rocks were partial melts of the ultramafic, noted the pervasive dyke complex and suggested that
the massive positive gravity anomaly is caused by the massif lying in the hanging wall of a major
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thrust. Ian Gass (1968), following a decade of detailed mapping, then suggested that the Troodos
Complex is a fragment of oceanic crust and mantle, thus bypassing a host of bizarre and non- actualistic origins. Hess (1955,1962) had drawn attention to the common occurrence of two belts of ophiolite in orogenic belts, well-displayed in Newfoundland. Most workers (e.g. Moores and
Vine, 1971; Dewey and Bird, 1971b) considered ophiolite complexes to have been formed either at oceanic ridges or in back-arc basins, until Miyashiro (1973) pointed out that ophiolite basalts commonly have calc-alkaline arc affinities. This led to the general recognition that boninites (high- silica and magnesium, wet, high-temperature, low-pressure andesites) are almost ubiquitous in ophiolite complexes, which prohibits their generation at ridges or in back-arc basins. Boninites are common in fore-arcs and this led to the idea that ophiolites originate in new fore-arcs where a fracture zone converts to a subductionDraft zone and the subducting plate rolls back to leave an extensional zone filled by an ophiolite. This seems to be unsatisfactory explanation because, although roll-back is common in extant subduction zones, it does not generate fore-arc spreading bur rather arc-splitting to form new back-arc basins (Karig, 1972). Dewey and Casey (2011, 2013) developed an alternative model, using data mainly from the southwest Pacific, for ophiolite generation in the fore-arcs of oceanic arcs involving the nucleation of a trench on an oceanic transform. Transform to subduction conversion probably does not happen spontaneously but where a transform become transpressional during progressive plate boundary evolution around an instantaneous pole or when stress fields suddenly change during continent-continent or arc- continent collision (Dewey 1975). Thus a ridge to ridge transform and its fracture zone continuations are converted to a subduction zone. One ridge is progressively subducted but the other hanging-wall ridge, normal to the trench, continues spreading between the resultant upper two plates in a direction parallel with the trench. Hence, oceanic crust and mantle with a boninite-
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rich (high temperature, low pressure, wet, high magnesium and silica andesite) is generated in an
ophiolitic fore-arc that elongates with the arc. Hall (2002) generated a comprehensive plate
tectonic model for south east Asia, which gave us the key insight of arc elongation. A fore-arc
origin also aids the eventual obduction of a thin ophiolite sheet provided that the necessary arc-
continent collision occurs sufficiently soon after ophiolite generation, say 20 my when the
ophiolite lithosphere is still thin. If too long after generation, the ophiolite lithosphere will be thick
and bull-doze the margin rather than obduct. A key feature of the model is that the metamorphic
sole is generated from MORB in the subducting slab at about ten kilobars and becomes attached
to the base of the ophiolite prior to or during obduction. This involves flattening of the subduction
angle and the elimination of the asthenospheric wedge. Dewey and Casey (2013) also attempted
to relate the shear directions and sensesDraft of subduction and obduction melanges in the lowermost
levels of the Bay of Islands Ophiolite Complex to a pre- and syn-obduction fore-arc plate
configuration with a trench-trench-ridge triple junction.
Baragar (1953 Falconbridge unpublished report) made an original and clever discovery and
interpretation at Nippers Harbour in the Betts Cove Ophiolite Complex of Newfoundland long
before it was recognized as an ophiolite. He mapped large areas of dolerite and was puzzled over
why such a large intrusion had not cooled slowly to a gabbro. He then began to see contacts and
chilled margins within the dolerite mass and realized that it consisted of nothing but dolerite dikes,
all striking in roughly the same direction. He mused over why there are no “country rocks” into
which the dykes were intruded nor any non-dyke screens, and suggested that the dykes must fill
an extensional gap at least as wide as the extensional separation To my knowledge, this is the
second recognition and description of sheeted complex and its significance but, sadly, not
published and long before the significance of ophiolites had been discovered.. However, Arthur
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Reginald Daly, who was a rare North American mobilist (Daly, 1906, 1923, 1925a, 1933) made a similar discovery on St. Helena, close to the South Atlantic mid-ocean ridge. In 1921-22. he found a composite dyke consisting of two hundred mafic dykes. Daly posited that here was the answer to what filled the gap at the “trailing edge” of his sliding continents with, clearly, a direct understanding of dyke complexes that we now call sheeted complexes. This was a brilliant example of using and linking detailed field data to a large-scale theoretical tectonic concept with the open mind ready to use and understand that link.
Precambrian tectonics
Earth is the only terrestrial planet in the solar system with plate tectonics, although Venus, with roughly the same size and densityDraft of Earth, and Mars may have had plate tectonics but are now stagnant lid planets. Some of the icy moons of the giant gas planets are tectonically active and may have ice plate tectonics. Since Burke and Dewey (1973), I have read widely on and gone out of my way to take every opportunity to visit as many Pre-Cambrian terrains as possible, especially the Archaean. My field knowledge of Pre-Cambrian South America, East Asia, and
Antarctica remain scanty or non-existent but I have a working knowledge of most other Pre-
Cambrian terrains. By the early 1970s, it became clear that plate tectonics explains the tectonic behaviour of Earth for at least the last 0.6 Ga , the late Proterozoic Ediacaran and the Phanerozoic, an opinion held now by most geologists. It is said that one could not get a job in academe prior to about 1955 in North America if one accepted continental drift; Daly was a rare exception. The reverse was true after about 1970. However, the tectonics of Earth prior to 600 Ma is exceedingly contentious. The principal questions are “when did plate tectonics start and, if there was a time when plate tectonics was not operating, what was tectonics like on a single-plate (stagnant-lid)
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Earth”? Opinions have ranged widely for the time of the beginning of plate tectonics from just
after the collision of proto-Earth with a body called Theia that splashed out the Moon, to 0.6 Ga.
One of the traditional problems of studying the Pre-Cambrian is the lack of fossils that can be used
to date and correlate strata. Only recently has widespread zircon dating permitted new insights
into the correlation of Pre-Cambrian rocks and event timing.
It is my opinion, based mainly upon field observation and petrology, that plate tectonics
began during the late Palaeo-Proterozoic at about 2.2 Ga. (Dewey, 2018) as outlined by Burke and
Dewey (1973). The basis of viable tectonic interpretations lies in adherence to the direct observed
mapped geology of a region, depicting structure, rock assemblages, and their relationships, not
inferences based upon geochemistry and modelling, upon which there has been too much over-
reliance, especially in tectonic interpretationsDraft of Archaean rocks. Figure 5 (adapted from Dewey,
2018) is a schematic table of events and derived interpretations. 1-8 are proposed tectonic phases
of Earth’s evolution. I have been impressed by the very great differences in the map patterns and
structures of terrains of successive phases. Little can be said about the Hadean because the only
physical remnants are detrital Hadean zircons. The first clear preserved continental crust, the
ancient grey gneisses, was generated during the Eo-Archaean Late Heavy Bombardment, probably
by foundering of a Hadean mafic crust. During the Palaeo-Archaean, thick komatiite build-up on
the Ancient Grey Gneiss basement was followed by tectonic inversion to develop tonalite-
trondjemite-granodiorite TTG domes with intrusive to structural contacts against greenstone keels
characterized by axial vertical prolate stretching. The Neo-Archaean is enigmatic and is regarded
by many as an accumulation of collided primitive oceanic arcs. I have strong reservations about
this view. Many evolve to silicic assemblages and have at least in places, an older TTG gneissic
basement. The first linear/arcuate deformation belts, such as the Limpopo, appear at about 2.0 Ga
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and the first clear evidence of the operation of the Wilson Cycle is the Coronation-Wopmay Belt.
The Birrimiam of west Africa may be the first assemblage of islands arcs on Earth. The arguments for a 2.2 beginning of plate tectonics are fully developed in Dewey (2018). In rocks older than 2.2
Ga, I can find no evidence of the key geological indicators of plate tectonics that are present after
2.2 Ga and are obvious and pervasive from about 0.6 Ga. These are subduction zones, sutures, island arcs, rifted continental margins, and ophiolites. Palaeomagnetism (Evans and Pisarevsky,
2008) demands large lateral relative motion among continents back to about 1.88 Ga and permits it back to about 2.7 Ga. Certainly, there was a tectonic revolution from about 2.4 Ga with a large amount of mafic magmatism. The 2.1 Ga Jormua Ophiolite Complex of Finland is the oldest known. Draft
Plate tectonics and geology: the quantitative approach
The ground-breaking paper by Tanya Atwater (1970) was the key step in the development of a quantitative predictive relationship between plate tectonics and geology, which she had presented at the Asilomar Penrose in 1969. Tanya generated a complete plate kinematic model for the East Pacific, tied it to the present plate boundary configuration of the Pacific margin of North
America, and ran the plate boundary configuration back to 40 Ma. The result was breath-taking in the way that it explained the broad Cenozoic geology of the North American Cordillera and its timing. While the Farallon Plate separated the Pacific Plate from North America, a continuous subduction zone with its attendant volcanic arc ran from Alaska to Mexico. The motion of the
Pacific Plate with respect to North America was roughly parallel with the subduction zone so that the Pacific-Farallon Ridge gradually approached the trench. Because the Mendocino Transform offset the Pacific-Farallon Ridge, the Ridge first collided with the trench at the Mendocino-Ridge
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corner, following which the ancestral San Andreas Fault system grew in length between the
Mendocino TFF triple junction and the Gulf of California FRT triple junction. As the San Andreas
Transform boundary lengthened , the subduction zone and arc shut off diachronously and a
triangular slab window grew. This paper was the first to show the rigorous predictive power of
plate tectonics in explaining a continental geological pattern.
Smith (1971) used the relative motion of Africa with respect to North America from about
160 Ma and the Cenozoic motion of Europe with respect to North America to define the motion
of Africa relative to Europe by finite rotations around successive poles using the finite differences
in successive continental reconstructions. From this, he showed that the path of Africa with respect
to Europe can be split into three distinct phases: 1 in which Africa moved east-south-east, 2 to the
west-north-west and 3 to the north. He correlatedDraft this relative motion, convincingly, to the broad
tectonic evolution of the Alpides from Iberia to the Black Sea through the Mesozoic and Cenozoic.
During phase 1, extension generated rifts, troughs and narrow para-oceans in the western Alps and
Dinarides, in phase 2, subduction and blueschist metamorphism occurred, and, in phase 3, major
shortening and nappe formation developed in the Alps.
Dewey et al (1973) used Smith’s (1971) finite difference technique to derive a series of
finite Africa-Europe rotation poles but, from the Atlantic data of Pitman and Talwani, used
successive magnetic anomaly pairs to define an Africa-Europe relative motion path with many
more and much shorter steps. This allowed the determination of a far more accurate path and its
relationship with the evolving geology of the Alpides in far greater detail. Rotation poles are of
three kinds (Dewey, 1975) and should not be confused. First, a pole and angle may be constructed
to move a rigid body on the surface of a sphere from one to another position, such as fitting two
congruent rifted continental margins. This is a finite difference pole, which, for a very short time
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interval, such as magnetic anomaly pair fitting, may approximate the true relative motion but, for a long period of time, is merely a finite difference pole of convenience and is unlikely to be the finite motion or instantaneous pole that describes the true motion. A finite motion pole is defined as one in which the pole is fixed with the two plates whose motion it describes. An instantaneous pole of rotation is defined as one that moves in the frame of reference of the two plates whose motion it describes. The significance of this distinction is outlined in a later section.
The principal mistake in Dewey et al (1973), in addition no doubt to others, is that we believed the main giant west-closing wedge of the oceanic tract of Palaeo-Tethys between Africa and Europe to have been subducted and closed along the Black–Caspian Sea zone, as early Jurassic oceans opened in the Alpine and Vardar Troughs and Eastern Mediterranean. Sengor (1979b) pointed out that the Black-Caspian Sea Draftzone is a remnant of a Cretaceous back arc basin, and that
Palaeo-Tethys closed within the Pontides’.
In the mid-seventies, I did some consulting for Philips Petroleum to develop a model for the evolution of the Caribbean; from this, it became clear that the Caribbean Plate is mostly of
Pacific origin and had slid eastwards through the gap between North and South America. Based upon this, Jim Pindell and I undertook a more detailed study of the Caribbean and Gulf of Mexico
(Pindell and Dewey,1982), which Jim then fully developed as his Ph.D thesis. Today, Jim is distinguished for his profound knowledge and understanding of the Northern Andes, the
Caribbean, and the Gulf of Mexico. In the early eighties Jim and I worked on a finite difference method, using slip vectors and boundary orientation to develop the relative motion and rotation of blocks in a plate boundary system, based upon northwest South America and Turkey (Dewey and
Pindell, 1985).
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In Albany, I spent a great deal of time working on the theoretical kinematics of global plate
mosaics, not only to understand their complex evolution but to provide a potential basis for the
origin and development of geological patterns, structures, and histories of rock masses along plate
boundary zones and at triple junctions. This, in my view, is the holy grail of structure and tectonics
and came to partial fruition in Dewey (1975) in a special volume of the American Journal of
Science to honour John Rodgers. This was a mistake only in the sense that the special volume is
hard to access and almost part of the “grey literature” to the extent that the paper and its message
became somewhat of a “sleeper” even to this day. Therefore, because the message is subtle,
complex, important, and not widely understood, I feel it useful to outline its essentials again briefly
here. I use equal-angle stereographic projections to plot a variety of theoretical plate configurations
of which I present and discuss three hereDraft (figs. 2, 3, and 4). They may be visualized as either upper
or lower hemisphere; upper is preferable if one wishes to view Earth from outside rather than
inside. In the figure 2 three-plate system, plate a is fixed in the stereographic reference frame.
Poles ba and ac are fixed in the a frame of reference and, with the angular velocity of rotation
around them, describe the relative motion of plates b and c with respect to plate a. The motion of
plate b is anti-clockwise and c anti-clockwise with respect to a. Poles ba and ac are termed finite
motion poles because all points on plates a and b retain the same angles with pole ba, and points
on plates a and c retain the same angles with pole ac. Very simple ba and ac ridge-transform
boundaries evolve around fixed finite motion poles. The bc and ac angular velocities of rotation
are the same. In contrast, pole bc, which lies on an ac-bc-ba great circle midway between ac and
ba and describes the relative motion between plates b and c, cannot be a finite motion pole. It can
be only an instantaneous pole of rotation because all points on plates b and c constantly change
their angular position with respect to pole bc. Therefore, pole bc is written as bci. The position of
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bci is fixed, with respect to ac and ba, on the ac-ba great circle as the point at which points on ac and ba circles are moving at the same velocity where they are tangent as they cross the ac-ba great circle This is not a trivial difference because it means that, whereas the slip vector is constant in magnitude and direction for plate boundaries evolving around finite motion poles, the slip vector must change in magnitude and direction at boundaries evolving around poles that are only instantaneous because both plates b and c are moving in the bci reference frame. Finite difference poles are used to define a “rotation of convenience” such as making a restoration between pairs of co-eval magnetic anomalies or congruent margins. For large rotations, they are unlikely to represent a true rotation path.
We now follow the evolution of the simple three-plate mosaic of figure 2, particularly the evolution of the abc triple junction and theDraft bc boundary concentrating on the bc boundary because the evolution of the ab and ab finite motion boundaries are simple except where the bc boundary is converted diachronously and locally into an ab boundary. 1-5 are a series of reference localities on the b hanging wall of the bc boundary and all plate boundaries are portions of small or great circles. At t1, orthogonal subduction along the bc boundary increases steadily in rate from the bci pole to the triple junction. From t1 to t2, the bc trench slip vector becomes progressively oblique.
The bc ridge has one transform offset. The triple junction is RRT. After 20° of rotation around ac and ba, we have the configuration at t2. The triple junction has migrated along the the bc subduction zone to a position between 3 and 4 and changed to TTR. Between 5 and 3, the slip vector is now dextral oblique along an ab finite motion trench. 1 has moved through bci into a field of slow extension, and new short transpressive transforms allow the continuation of orthogonal spreading. From t2 to t3, the triple junction migrates, with ridge subduction, as a TTR until the closest ac transform arrives at the trench, whereupon it becomes TTF and migrates back to 5.
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Points on the bc subduction zone continue to move, diachronously, through the bci pole region
into the extensional field. The transforms on the bc ridge become more transpressive; the long ones
are likely to become sites of subduction nucleation (Casey and Dewey, 1984). Between t3 and t4,
the triple junction has again migrated along the trench as a TTR to about 3 and then migrated back
to 5 as a TTF. Thus four sudden changes in the rate and direction of slip occur along the hanging
wall of the subduction zone; this is one way to account for polyphase deformation. Therefore, a
fairly simple plate mosaic can yield a very complicated evolution of triple junction migration,
sudden change, and evolution; slip directions and rates may change progressively or very suddenly.
Lastly, evolution about an instantaneous pole must cause complexity in the evolution of ridges and
fracture zones.
The kinematic evolution illustratedDraft in Figure 2 has two finite motion poles and one
instantaneous pole on whose plate boundary all the complexity damage is inflicted. A simple
construction (Figure 3) gives a three instantaneous poles in a three plate system whose boundaries,
more realistically, share the damage. In this, I use the stereographic projection (proxy for the
mantle) as the reference frame, and plot three finite motion poles (ma, mb, and mc) that describe
the motion of plates a, b, and c in this reference frame, 120° apart and with the same angular
velocity. Thus, we can define three instantaneous poles (cai, abi and bci), 120° apart and with the
same angular velocity that describe the relative motions among plates a, b, and c, Thus a, b , and
c must move with respect to their instantaneous pole set All six poles lie, of course, on the same
(equatorial) great circle. From the t1 starting plate boundary configuration of figure 3a, we evolve
it by making three successive finite rotations of 20°around ma, mb, and mc to give the
configurations of t2, t3, and t4. A, B, and C are hotspots fixed in the mantle (stereographic)
reference frame and the dotted lines are the resultant hotspot tracks. Figure 4 illustrates a similar
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boundary configuration differing, in the starting configuration, only in the existence of a continental block, and the subduction polarity along the ab boundary. 1-7 are reference points.
These differences engender a startling contrast between the evolution of the two configurations. In
3, the ab transform elongates as one end becomes increasingly transpressive and the slip direction oblique such that it is converted to a subduction zone. The other end becomes transtensive and a short ridge segment develops perhaps like the Cayman Trough. Magnetic anomaly pairs generated at the ac and bc ridges become non-copolar with the ridge. The transform adjacent to point 6 becomes a subduction zone and elongates as a transform to join the ac ridge. Large areas of b are subducted beneath a. In figure 4, the continent collides with the ab arc and subduction polarity flips. An elongating finger projection of a invades b; its boundary with the continent is constrained, with transpression, until the arc clears theDraft continent, when it is then able to follow an ab transform.
Large areas of a and c are subducted beneath b along the 2-3 subduction zone.
The essential lesson is that, although plate tectonics provides a rational framework for the tectonic evolution of the geology of the lithosphere, the evolution of plate boundary mosaics inherently yields very great temporal and spatial complexities. If we suppose that the evolution of rock masses in plate boundary zones is related to the direction and rate of slip across them, plate tectonics provides a rational basis for understanding their complex geology and evolution. While it is easy, as here, to forward model the detailed kinematic evolution of, and theoretical history of rock masses in plate boundary zones, it is a formidable task to derive the unique evolution of a plate boundary system from observed geology, which is commonly polyphase and complex. The principal causes of complexity are progressive and sudden changes in triple junction evolution, the gradual and sudden changes in the slip vector across plate boundaries, and the myriad of ways in which the slip vector is partitioned within plate boundary zones especially in wide continental
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boundary zones. In general, pole sets are instantaneous, and slip directions and fracture zones
usually change orientation gradually but, occasionally, sharply. Long term finite motion poles are
uncommon except, perhaps, where oceanic transforms and fracture zones are very long and cut
and bound thick lithosphere thus acting as “constraining guides”, as between Brazil and Ghana.
One could say that plate tectonics makes geology as complex as we know it is but the kinematic
and dynamic relationship between plate boundary kinematics and the evolution of rock masses
has, as yet, been little explored.
Another paper (Dewey, 1980), which I wrote in Calgary in 1979, suffered a similar “fate”
of being buried in a Geological Association of Canada Special Paper, This explores the causes
and effects of the relative motions of upper and lower plates and the mantle in subduction zones.
Its conclusion is that the geology of subductionDraft zones is, principally a consequence of the motions
of the upper and subducting plates in a mantle frame of reference based upon the notion that a
large subducting slab area blocks mantle flow and that a slab cannot advance or retreat horizontally
through the mantle but is easily “inserted” into the mantle down slab dip and can slowly sink
vertically. In this model, subduction is of three types: down-dip slab insertion, vertical
gravitational sinking to generate slab roll-back, and slab override and flattening by the upper plate.
I ascribe velocities as negative where motion is away from the trench line and positive when
toward: Vo and Vss-velocities of upper plate normal to and parallel with the trench line
respectively, Vs-velocity of subducting plate horizontally prior to subduction and down slab, Vg-
vertical gravitational sinking of subducting slab, and Vr –roll-back. The simplest arrangement is
Vo=0 without or with Vss, and Vr=0, which gives a neutral arc without or with strike-slip faulting
in the upper plate, most likely along the arc volcanic zone; subduction-accretion is common in
such arcs as in the Mentawei Islands and Alaska. The Caribbean is a fine example of a plate trapped
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in the mantle reference frame by two neutral subduction zones with Cost Rica and Barbados accretionary prisms as the North and South American Plates move westward with respect to the
Caribbean with frontal orogenic systems. Where –Vo or where +Vo
Durham and Oxford Draft
From 1981 in Durham and Oxford, I moved back into Irish geology with Donny Hutton and Paul Ryan, mainly the structure of the South Mayo Trough, the origins of polyphase deformation, and the tectonic evolution of western Ireland. I became interested in the Cenozoic tectonics of Ireland and concluded that much of the topography and faulting there and in the British
Isles generally is of Cenozoic origin (Dewey, 2000). The Scottish Highlands are a dissected dome tableland with Paleocene bimodal magmatism and faulting to the west forming the beginning of the Iceland nematath. North-south shortening in southern England, related to shortening in the
Alps and/or Eocene collision in the Pyrenees, extends west to the Bristol Channel; David Peacock,
David Sanderson, Tony Watts and I are investigating the topography of Exmoor as the hanging wall of a, possibly, Miocene thrust. In Ireland, topography is fragmented, not from resequent emergence of resistant Palaeozoic rocks from beneath a degrading cover of Carboniferous
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limestone but mainly by the Cenozoic rotation, tilting, and uplift of normal fault blocks (Dewey,
2000).
In 1985, I had the privilege and pleasure of spending four months on a south to north
geotraverse of the Tibetan Plateau led by Robert Shackleton and Chang Chengfa, and sponsored
by the Royal Society and the Chinese Academy. Our principal aim was to investigate the Cenozoic
structure and evolution of the Plateau, especially its mechanism of uplift. The Plateau has an 80
km thick crust and we were expecting but failed to find field structural evidence of north-south
shortening to account for the thick crust and plateau uplift. GPS data (Zhang, 2004) clearly shows
north-south shortening with velocities relative to stable Asia steadily reducing northwards across
the plateau from 32mma-1 to 2mma-1, while gradually swinging from N-S to E-W. The area
between the Himalayan Boundary ThrustDraft and stable Asia has to accommodate the northward
motion of India by about 2,500 km (Dewey, Cande, and Pitman, 1989). Perhaps the shortening is
accommodated by pop-down wedge tectonics (Cagnard, 2006) . The common small-scale
conjugate strike-slip faulting could account, at least in part, for the north-south shortening but not
crustal thickening. Lithospheric delamination (Dewey, 1988) and doubling crustal thickness by the
underthrusting of greater India (Argand, 1924) have been suggested for Plateau uplift. Even though
the velocity field shows a northward-increasing component of west to east motion on the Plateau,
wholesale eastward extrusion is denied by the orthogonality of the velocity field to the southern
boundary The Plateau remains even more of an enigma since high velocity mantle has been
discovered beneath it. I have seen no complete synthesis of the Plateau that convincingly explains
its principal features and their origin.
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Davis
Retiring from Oxford in 2001, the wanderlust struck again and I accepted a position at UC
Davis and was introduced by David Benner, Eldridge Moores, Rob Twiss, John Wakabayashi, and many others to whom I am deeply grateful, to some of the finest active tectonic geology in the world. The world of neotectonics was to occupy much of my eight years in Davis and, through teaching an undergraduate course, I became fascinated with natural hazards, especially floods, tsunamis, and landslides Figure 5 illustrates a highly simplified neotectonic map of California and adjacent areas of interest. and I was especially interested in the 3 ma to present East California
Shear Zone from Lake Tahoe to the Garlock Fault within which my students, David Benner and
Tatia Taylor, did a superb job of mapping the Coso area (Fig. 5) both a US Navy Weapons Testing
Area and Navy Commercial GeothermalDraft area, the latter managed by Frank Monastero, also our co-researcher and co-author. We were helped greatly by Angela Jayko who, for many years mapped, in great detail, a huge area between Owens Lake and Coso. The East California Shear
Zone is a 300 km long and 30 km wide transtension zone within which is a diverse “treasure house” of geological features both recent and active, namely small transpressional areas such as the
Poverty Hills, dominantly strike-slip and extensional faults, small thrusts and folds, at least one incipient extensional detachment, block rotation (from palaeomagetism), silicic volcanic cones, ignimbrites (Bishop Tuff), basalt flows, Owens Lake, Lake Tahoe, Mono Lake, and the Long
Valley Caldera. Our research involved mapping of bedrock, young sediments, terraces, geomorphology, and structure. We showed that, from the pristine granitoids in the high Sierra
Nevada, there is a progressive structural and hydrolytic degeneration and subsidence to a complex array of rotating fault blocks of diminishing size. We related this to the complex interaction of normal and strike-slip faults to generate a triaxial bulk strain in the brittle field during transtension
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supported by a GPS-derived velocity field and bulk strains from small earthquake swarms.(Dewey,
Taylor, and Monastero, 2008, Taylor, Dewey, and Monastero, 2008).
The Coso work also fanned my interest in the relationship between relative plate
displacements, velocity fields, bulk strains, and rotations, especially in New Zealand, and the
Aegean and Turkey. I am continuing work on these topics and areas but it is already clear that bulk
strains in plate boundary zones are, in general triaxial, in transtension or transpression, and are
rarely biaxial or uniaxial. Non-coaxial biaxial strains are common in small-scale shear zones,
prolate in Archaean greenstone belts, and oblate in the basins of continental FFF triple junctions.
“Retirement”
Retirement is a euphemism for havingDraft the time to do much more research. I am continuing
with my interests in Newfoundland, Ireland, ophiolites, the geology of triple junctions, outcrop-
scale shear zones, the Pre-Cambrian, the Exmoor-south Dorset shortening zone, and the general
geology of the British Isles but I have also started work on the boulder deposits of tsunamis and
storm waves. Catastrophic sedimentary events have long fascinated me after reading Ager (1973)
and teaching a natural hazards course in UC Davis. Paul Ryan and I have been working on storm
deposits in western Ireland and tsunami deposits in the Miocene of North Island, New Zealand
(Dewey and Ryan, 2017). We are developing the idea that many turbidites in subduction zone
basins are the products of tsunami backwash. I am also gradually writing my memoirs.
With Jack Casey, I have rekindled my interests in the evolution of western Newfoundland,
especially Ordovician ophiolite obduction and the deformation and metamorphism of subjacent
metamorphic complexes. Figure 7 shows, schematically, sections of the Laurentian margin of the
Appalachian Caledonian Belt, each illustrating the somewhat differing basic geometry of of a
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particular area. Each involves the collision and obduction of an Ordovician arc and ophiolitic fore- arc with the Laurentian continental margin followed by a flip in subduction polarity. The collision/obduction contact is seen in the steeply-dipping Highland Boundary Fault , Clew Bay
Zone and Baie Verte Line containing, variably, fragmented ophiolites, continental margin lherzolites, mafic volcanics, and blueschists. The Newfoundland section (A) is especially complete and instructive. A single sheet of arc (Lushs Bight) and ophiolite (Bay of Islands, Betts Cove, and
Point Rousse Ophiolite Complexes) with a younger arc volcanic and volcaniclastic sequence
(Snooks Arm and Flatwater Pond Group) collided with and was obducted onto the hyper-extended
Laurentian margin at about 475-470 Ma. The obduction contact was flat and shallow and was inherited from the pre-collision subduction plate boundary. However, another subduction contact exists with the Fleur-d-Lys metamorphicDraft complex and its basement, where De Wit (1972) identified Neo-Proterozoic greenschist and garnet amphibolites (Fleur-de-Lys), deformed at about
475-459Ma, forming a carapace to an eclogite-facies Grenville basement. The eclogites are
Ordovician (c. 475 Ma, De Wit and Armstrong, 2015); their contact with the Fleur-de-Lys is a massive shear zone with both top to the west and later top to the east shear sense indicators. This suggests that the shear zone operated as a second deeper subduction zone that detached the basement from the Fleur-de-Lys cover. The basement was first subducted, then educted, along the shear zone. The area would benefit from a major structural study to test this idea.
Lastly, with Jean-Pierre Brun and Frederick Gueydan, I have developed an interest in the structure, layer strength, and integrated strength of the continental lithosphere, a central problem in structure and evolution of orogens. It is inappropriate, here, to delve into the details of our work but our principal conclusion is that the lithosphere is generally strong but with great lateral and vertical variations that engender great variations of tectonic styles in plate boundary zones. We
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suggest that the controversy between strength residing principally in the mantle with a weak lower
crust (Watts and Burov, 2003) versus in the upper crust with a weak mantle (Jackson, 2002) is a
false tussle in that these are merely two patterns in a wide spectrum. However, we lean, generally,
towards a strong mantle away from plate boundary zones for many reasons including the need for
a torsionally strong lithosphere to allow plate tectonics, the clear evidence for long wave length
buckling, the long wave-length flexure of the lithosphere with the support of large loads as in the
Himalayas, and the preservation of substantial gravity anomaly pairs in old orogens. Weakening
comes with mantle hydration, deformation, smaller grain size, and high heat flow, thereby
conveniently weakening the lithosphere and aiding its deformation, mostly in convergent plate
boundary zones. Draft
Conclusions
In conclusion, my advice to the young aspiring geologist is: learn your basic geology well
and broadly, and see and map thoroughly as much as you can in the field around the globe. Read
as much as you can across the earth sciences spectrum, not just the literature of the past five years
but deeply into the archives, in this way you will avoid “rediscovering the wheel” especially in
elliptical or rectangular form. Go to all the lectures and seminars that you can, and travel to national
and international meetings where you will meet many other earth scientists from around the world
and learn new ideas. Spend part of your career abroad including North America. By all means
develop a speciality into which you can dig a deep hole and develop a great understanding but it
is important to put that understanding into a broader context. However, sometimes, one digs a hole
so deep that one cannot see out. Rather, perhaps, dig lots of holes between which one can see
relationships. Much of geology suffered, before the development of plate tectonics, by having no
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framework in which to put and understand a lot of detail. Modern geology is integrative and needs to embrace field and laboratory data, every kind of modelling from numerical to analogue and kinematic, experiment, the analysis and synthesis of data, remote-sensing, and data and ideas from all the other sciences and mathematics. Think at all scales from the microscopic to global but always remember the words of Francis Pettijohn that “the truth resides in the rocks” and that “there is nothing as sobering as an outcrop”. Above all, avoid side-lining yourself into administrative pursuits especially those of government where you may receive honours and gongs but where your contributions to science will atrophy. Stay with geology, the most collegial and rewarding of sciences, and have a lifetime of valuable enjoyment in teaching and research. Honour those who, over two hundred years, have laid the foundations for our present understanding. Demand and contribute high and ethical standards. Finally,Draft try to be generous, open, encouraging, and kind, especially to students, who will carry the science forward.
Dedications and Acknowledgments
This paper is dedicated to my friend and colleague the late Kevin Burke whose eclectic and agile mind, and research in many areas of tectonics, led to a deeper understanding of Earth. Also,
I remember those ground-breaking geologists Harry Wellman and Bert Quennell whom I knew for short times. I thank my fifty six graduate students from 1960 to 2012, who taught me so much. I acknowledge, with gratitude, the influence of very many geologists, especially the following:
Tanya Atwater, Derek Flinn, John Hayward, William Kennedy, Jack Kirkaldy, Charles Lapworth,
Win Means, George Plafker, John Ramsay, Nicholas Rast. Paul Ryan, Robert Shackleton, Douglas
Shearman, Alan Smith, Cees Van Staal, Don Turcotte, George Walker, Tuzo Wilson, and Dennis
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Wood. Paul Ryan, Tim Kusky, and Celâl Şengör made valuable suggestions for improving the
manuscript. My debt to Celâl is expressed in the introduction to this volume.
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Figure descriptions
Fig. 1. A transform-transform-shortening zone in continental lithosphere such as the Karliova triple junction in eastern Turkey (Dewey et al, 1986). Discussion in text.
Fig. 2. Upper hemisphere equal-angle stereographic projection of a three plate mosaic with two finite motion poles and one instantaneous pole. Discussion in text
Fig. 3. Upper hemisphere equal-angle stereographic projection of a three plate mosaic with three instantaneous poles. Discussion in text Draft Fig. 4. Upper hemisphere equal-angle stereographic projection of a three plate mosaic with three instantaneous poles and a continental non-subductable continental area . Discussion in text
Fig. 5. Table illustrating the tectonic evolution of Earth. Discussion in text.
Fig. 6. Schematic neotectonic map of California. Discussion in text.
Fig. 7. Schematic sections of the Ordovician Appalachian-Caledonian continental margin of
Laurentia. Discussion in text. Green-basement (G-Grenville, L-Lewisian), orange-allochthonous
Cambro-Ordovician clastics, blue-autochthonous Cambro-Ordovician clastics to carbonates, yellow-Neoproterozoic to Ordovician mainly clastic Barrovian metamorphic complex, yellowgreen- accretionary complex, red- ophiolite and island arc, 1-shear sense footwall down, shear sense footwall up, BC-Ballantrae Complex, BCOC-Betts Cove Ophiolite Complex, BL-
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blueschist, BOICC-Bay of Islands Ophiolite Complex, BVZ- Baie Verte Zone, CA-Connemara
Antiform, CBZ-Clew Bay Zone, CC-Coastal Complex, DA-Dalradian, EC-eclogite, FL-Fleur de
Lys, G-Grenville, HBF-Highland Boundary Fault, LAS-Loch Awe Syncline, Lh+o-lherzolite and
ophiolite, LTN-Loch Tay Nappe, MOT-Moine Thrust, MT-Mannin Thrust, MV-Midland Valley,
Ph-phengite, PS-Portaskaig Slide, RIL-Red Indian Line, SC-South Connemara, SF-Southern
Uplands Fault, SMT-South Mayo Trough.
Draft
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A
An
E
DraftE An A
Karliova
dextral
sinistral
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Draft
Figure 2. 2. Upper hemisphere equal-angle stereographic projection of a three plate mosaic with two finite motion poles and one instantaneous pole. Discussion in text
429x430mm (300 x 300 DPI)
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Draft
Figure 3. 3. Upper hemisphere equal-angle stereographic projection of a three plate mosaic with three instantaneous poles. Discussion in text
456x465mm (300 x 300 DPI)
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Draft
Figure 4. 4. Upper hemisphere equal-angle stereographic projection of a three plate mosaic with three instantaneous poles and a continental non-subductable continental area . Discussion in text
442x450mm (300 x 300 DPI)
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Draft
Figure 5. Table illustrating the tectonic evolution of Earth. Discussion in text.
232x263mm (300 x 300 DPI)
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Draft
Figure 6. Schematic neotectonic map of California. Discussion in text.
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Draft
Figure 7. Schematic sections of the Ordovician Appalachian-Caledonian continental margin of Laurentia. Discussion in text. Green-basement (G-Grenville, L-Lewisian), orange-allochthonous Cambro-Ordovician clastics, blue-autochthonous Cambro-Ordovician clastics to carbonates, yellow-Neoproterozoic to Ordovician mainly clastic Barrovian metamorphic complex, yellowgreen- accretionary complex, red- ophiolite and island arc, 1-shear sense footwall down, shear sense footwall up, BC-Ballantrae Complex, BCOC-Betts Cove Ophiolite Complex, BL-blueschist, BOICC-Bay of Islands Ophiolite Complex, BVZ- Baie Verte Zone, CA- Connemara Antiform, CBZ-Clew Bay Zone, CC-Coastal Complex, DA-Dalradian, EC-eclogite, FL-Fleur de Lys, G-Grenville, HBF-Highland Boundary Fault, LAS-Loch Awe Syncline, Lh+o-lherzolite and ophiolite, LTN-Loch Tay Nappe, MOT-Moine Thrust, MT-Mannin Thrust, MV-Midland Valley, Ph-phengite, PS-Portaskaig Slide, RIL-Red Indian Line, SC-South Connemara, SF-Southern Uplands Fault, SMT-South Mayo Trough.
267x188mm (300 x 300 DPI)
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